Add press_any_key() function

Displays "Press any key to continue..." in shell terminal mode and waits
for a key stroke.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -882,6 +882,7 @@ void         psleep (unsigned);
 int          fork_exec (int *, int *, const char *, char *const *);
 int          shell_exec (int *, int *, char *);
 int          child_wait (int *, int *, int);
+void         press_any_key (void);
 
 /* vars.c */
 extern int               col, row;